Grounding

Use this information at your own risk. Do not tamper with mains AC voltages, unless you know what you're doing. Mains electricity can be lethal to people and to equipment.

There are three types of ground (or earth) which need managing. Earth ground comes from the mains AC wall socket. Analog and digital grounds are low power DC and are also used for signal ground. These come from the low voltage side of the PSU that's powering the modules. Analog ground for audio circuits and digital ground for powering any computer or logic systems.

The mains has three connections, live, neutral and earth. Do not confuse neutral with earth and do not connect these two together anywhere. To reduce noise always avoid eath loops and use star or short lengths of bus-bar ground wiring and single links to further star wired earth connections.

If the transformer comes with a screen winding it should connect to the point where mains earth is connected. If it has no screen winding make sure there is a good connection between the transformer and the chassis (any metal case parts, mounting rails, panels, etc.) at one point. Mains earth only connects in one place to the metal chassis (ideally the transformer mounting), and to any other isolated metal areas (front panels), this is for shielding from external noise, and to reduce the risk to the user of electrical shock in case of a fault. With a wooden case metal panels should be linked by wire to each other and the nearest chassis ground, but only one connection each time to avoid forming loops - unless using a grounded metal mounting rail. Only connect the chassis to DC/signal ground if it isn't not going to have any other connection to mains earth.

Digital and analog grounds only connect to each other in one place, that is at the power supply, possibly via ferrite beads to filter high frequency noise from the analog circuitry.

Connecting both signal ground and mains earth to chassis can create ground loops and cause mains hum. However if the two are completely isolated there can also be noise and hum problems. Then the solution is to connect the signal/DC ground at only a single point, via a parallel 100 to 150 Ohm resistor and 100n/630V capacitor pair, to the earth ground.

Shielding on signal wires is sometimes connected at both ends to earth, eg. front panel sockets or patch cables where it carries the reference ground, with respect to the signal, and other times only at one end, eg. shielding on wires to front panel controls pots connecting only, either to the ground plane on the PCB or to the metal of the panel.